The overall goal of this research is to understand the sperm function regulation by calcium, cyclic nucleotides and ion channels at the molecular level. This proposal focuses on the roles of a sperm tail ion channel (CatSper1) we discovered. CatSper1 from human and mice is only expressed in testis. The protein in sperm is strikingly localized in the principal piece membrane. Mice deficient in CatSper1 show no gross phenotypes except that males are completely infertile. The cyclic AMP-induced calcium influx into sperm is deficient in CatSper1-null mice. Because of its restricted protein expression in sperm tail and because of its restricted knockout phenotype in male fertility, CatSper1 represents an excellent target for safe non-hormonal male contraceptives. Aim 1 will determine the physiological roles of CatSper1 in male fertility. CatSper1-deficient males will be examined in detail and the reason(s) for male infertility will be determined. Aim 2 will uncover the molecular mechanism underlying the CatSper1 function regulation during spermatogenesis and sperm maturation. Four hypotheses will be tested: a. the CatSper1 protein expression is regulated; b. the protein's plasma membrane localization is regulated; c. the protein is modified; and d. the composition of the channel core protein complex is modulated. Aim 3 will test the current model concerning the roles of the cyclic nucleotide-gated (CNG) ion channels in the sperm cAMP-induced calcium influx. Aim 4 will test the hypothesis that the CatSper1 channel mediates a "cAMP- calcium positive feedback loop" in that cAMP causes calcium influx through the channel and the resulting calcium increase, in turn, causes cAMP to rise. Results from this study will allow us to understand the ion mechanism underlying sperm motility and may lay the foundation for the future development of drugs for infertility treatment and for male contraceptives.